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11
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The chemical processes
of photosynthesis occurring independent of light is called dark reaction
...
The dark reaction is purely enzymatic and it is slower than the light
reaction
...
In dark reaction, the sugars
are synthesized from CO2
...
The
process is called carbon fixation or carbon assimilation
...
In dark reaction
two types of cyclic reactions occur
1
...
Hatch and Slack pathway or C4 cycle
Calvin cycle or C3 cycle
It is a cyclic reaction occurring in the dark phase of photosynthesis
...
The Calvin cycle
was first observed by Melvin Calvin in chlorella, unicellular green algae
...
Since the first stable compound in Calvin cycle is
a 3 carbon compound (3 phosphoglyceric acid), the cycle is also called as C3 cycle
...

1
...
Reductive phase
3
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Carboxylative phase
Three molecules of CO2 are accepted by 3 molecules of 5C compound viz
...
This reaction
is catalyzed by the enzyme, carboxy dismutase
3 CO2 +

3 Ribulose
diphosphate

Carboxy dismutase 3 unstable intermediate 6
carbon compound

The three molecules of the unstable 6 carbon compound are converted by the addition of 3
molecules of water into six molecules of 3 phosphoglyceric acid
...

3 unstable
intermediate 6 C
compound

+

3 H2 O

Carboxy
dismutase

3 phosphoglyceric acid

3 phosphoglyceric acid (PGA) is the first stable product of dark reaction of photosynthesis
and since it is a 3 carbon compound, this cycle is known as C3 cycle
...
Reductive phase
Six molecules of 3PGA are phosphorylated by 6 molecules of ATP (produced in the
light reaction) to yield 6 molecules of 1-3 diphospho glyceric acid and 6 molecules of ADP
...
This
reaction is catalysed by the enzyme, triose phosphate dehydrogenase
...
Regenerative phase
In the regenerative phase, the ribose diphosphate is regenerated
...
It involves the
following steps
...
Some of the molecules of 3 phospho glyceraldehyde into dihydroxy acetone
phosphate
...

3 phospho
glyceraldehyde
3 phospho
glyceraldehyde

Triose phosphate isomerase

Aldolase

Dihydroxy acetone PO4
(DHAP)
Fructose 1,6 diphosphate

+ DHAP

2
...
Some of the molecules of 3 phospho glyceraldehyde instead of forming hexose sugars
are diverted to regenerate ribulose 1-5 diphosphate
3 phospho glyceraldehyde

Ribulose 1,5 diphosphate

4
...
Erythrose 4 phosphate combines with dihydroxy acetone phosphate in the presence of
the enzyme aldolase to form sedoheptulose 1,7 diphosphate(7C sugar)
Erythrose 4 phosphate

+ DHAP

Aldolase

Sedoheptulose 1 ,7
diphosphate

6
...

Sedoheptulose 1 ,7 + ADP
diphosphate

Phosphatase

Sedoheptulose 7
phosphate

+ ATP

7
...
Ribose 5 phosphate is converted into ribulose 1, 5 diphosphate in the presence of
enzyme, phosphopentose kinase and ATP
...
The ribulose
monophosphate is phosphorylated by ATP to form ribulose diphosphate and ADP,
thus completing Calvin cycle
...
In Calvin cycle, 12 NADPH2 and 18 ATPs are required to fix 6
CO2 molecules into one hexose sugar molecule (fructose 6 phosphate)
...
In this cycle, the first formed stable
compound is a 4 carbon compound viz
...
Hence it is called C4 cycle
...
This pathway is commonly seen in many
grasses, sugar cane, maize, sorghum and amaranthus
...
The chloroplasts are dimorphic
in nature
...
These bundle sheath cells have chloroplasts
...
The
chloroplasts in mesophyll cells are smaller and always contain grana
...
The bundle sheath cells are bigger and look
like a ring or wreath
...

The C4 cycle involves two carboxylation reactions, one taking place in chloroplasts of
mesophyll cells and another in chloroplasts of bundle sheath cells
...
Carboxylation
2
...
Splitting
4
...
Carboxylation
It takes place in the chloroplasts of mesophyll cells
...

This reaction is catalysed by the enzyme, phosphoenol pyruvate carboxylase
...
Breakdown
Oxaloacetate breaks down readily into 4 carbon malate and aspartate in the presence
of the enzyme, transaminase and malate dehydrogenase
...

3
...
The CO2 is used in Calvin’s cycle in the sheath cell
...
The CO2 is
accepted by 5 carbon compound ribulose diphosphate in the presence of the enzyme, carboxy
dismutase and ultimately yields 3 phosphoglyceric acid
...

4
...
This reaction is
catalysed by pyruvate phosphokinase and the phophoenol pyruvate is regenerated
...
The C4 plants are more efficient in photosynthesis than the C3
plants
...


Crassulacean Acid Metabolism (CAM) cycle or the dark fixation of CO2 in succulents
CAM is a cyclic reaction occurring in the dark phase of photosynthesis in the plants
of Crassulaceae
...
It is the
third alternate pathway of Calvin cycle, occurring in mesophyll cells
...
Most of the CAM plants are succulents e
...
,
Bryophyllum, Kalanchoe, Crassula, Sedium, Kleinia etc
...
g
...

CAM plants are usually succulents and they grow under extremely xeric conditions
...
The mesophyll cells have larger number of
chloroplasts and the vascular bundles are not surrounded by well defined bundle sheath cells
...
The CAM
plants are adapted to photosynthesis and survival under adverse xeric conditions
...
But they are better suited to
conditions of extreme desiccation
...
Acidification
2
...
The stomata in CAM plants are open in dark and they allow free
diffusion of CO2 from the atmosphere into the leaf
...


Phosphoenol Pyruvate

+

CO2 +

H 2O

PEP carboxylase

Oxaloacetic acid +

H3PO4

The oxaloacetic acid is then reduced to malic acid in the presence of the enzyme
malic dehydrogenase
...

Oxaloacetic acid

+

NADPH2 +

Malic dehydrogenase

Malic acid +

NADP+

The malic acid produced in dark is stored in the vacuole
...

Deacidification
During day time, when the stomata are closed, the malic acid is decarboxylated to
produce pyruvic acid and evolve carbon dioxide in the presence of the malic enzyme
...
This is called deacidification
...

Malic acid

+

NADP+

Malic enzyme

Pyruvic acid +

NADPH2

+

CO2

T
he pyruvic acid may be oxidized to CO2 by the pathway of Kreb’s cycle or it may be
reconverted to phosphoenol pyruvic acid and synthesize sugar by C3 cycle
...

CAM is a most significant pathway in succulent plants
...
As the stomata are closed, CO2 cannot enter into
the leaves from the atmosphere
...
During night time, organic acids are
synthesized in plenty with the help of CO2 released in respiration and the CO2 entering from

the atmosphere through the open stomata
...


Comparison of the plants of C3 and C4 cycle
C3 Plant

C4 Plant

1
...


2
...


The efficiency of CO2 absorption at
low concentration is quite high and
hence, they are more efficient plants
...


The CO2 acceptor is Ribulose-1, 5diphosphate
...


4
...


Oxaloacetate (OAA) is the first stable
product
...


Plants show one type of chloroplast
(monomorphic type)
...
The chloroplast of
parenchymatous bundle sheath is
different from that of mesophyll cells
(dimorphic type)
...
Leaves show
Kranz type of anatomy
...


In each chloroplast, two pigment
systems (Photosystem I and II) are
present
...

Therefore, these are dependent on
mesophyll chloroplasts for the supply
of NADPH + H+
...


The Calvin cycle enzymes are present in
mesophyll chloroplast
...


Calvin cycle enzymes are absent in
mesophyll chloroplasts
...


8
...


The CO2 compensation point is 0-10
ppm CO2
...


Photorespiration is present and easily
detectable
...


10
...


The CO2 concentration inside the leaf
remains low (about 100 ppm)
...
The 13C/12C ratio in C-containing
compounds remains relatively low (both
13
CO2 and 12CO2 are present in air)
...
e
...


12
...
c
...
of CO2 per dm2 of leaf area per
hour
...
of CO2 per dm2 of leaf
area per hour
...
The plants are
efficient
...
The light saturation intensity reaches in
the range of 1000-4000 ft
...


It is difficult to reach saturation even
in full sunlight
...
Bundle sheath cells are unspecialized
...


15
...


In these plants, it is 30-45°C and
hence, they are warm climate plants
...


16
...


30 ATPs are required to synthesize
one glucose molecule
...
External factors
1
...
Any kind of artificial light such as
electric light can induce photosynthesis
...
The effect of light on
photosynthesis can be studied under three categories
...
Light intensity
Wolkoff (1966) found that the arte of photosynthesis is directly proportional to light
intensity
...

The high light intensity which fails to accelerate photosynthesis is called light saturation
intensity
...
The rate of photosynthesis is greater in intense light than in diffused
light
...

i
...
Sciophytes (Shade plants)
At a specific light intensity, the amount of CO2 used in photosynthesis and the
amount of CO2 released in respiration are volumetrically equal
...

At very high light intensity, beyond a certain point, the photosynthetic cells exhibit
photo oxidation
...


In

general, low light intensity favours stomatal closure and in turn reduced rate of
photosynthesis
...
Light quality (wavelength)
Photosynthesis occurs only in the visible part of the light spectrum i
...
, between 400
and 700 nm
...


The green light has minimum effect and photosynthesis cannot take place either in the
infrared or in the ultraviolet light
...
Light duration
In general tropical plants get 10-12 hours of light per day and this longer period of
light favours photosynthesis
...
Carbon dioxide
CO2 is one of the raw materials required for photosynthesis
...

But, it is also reported that very high concentration of CO2 is toxic to plants inhibiting
photosynthesis
...
Temperature
The rate of photosynthesis increases by increase in temperature up to 40 ºC and after
this, there is reduction in photosynthesis
...
The temperature requirement for optimum
photosynthesis varies with the plant species
...
Similarly photosynthesis
stops beyond 40-50 ºC in certain plants; but certain bacteria and blue green algae can perform
photosynthesis even at 70 ºC
...
Water
Water has indirect effect on the rate of photosynthesis although it is one of the raw
materials for the process
...
Water rarely acts as a limiting
factor for photosynthesis
...

5
...
The phenomenon of inhibition
of photosynthesis by o2 was first discovered by Warburg (1920) in green alga Chlorella and
this effect is known as Warburg’s effect
...


In plants, there is a close relationship between Warburg’s effect and photorespiration
...
In plants that show Warburg’s effect, increased O2 concentration result in
diversion of these intermediates of Calvin cycle into the synthesis of glycolate, thereby
showing higher rate of photorespiration and lower photosynthetic productivity
...
Mineral elements
The elements like Mg
...

7
...
In 1929, Emerson found direct
relationship between the chlorophyll content and rate of photosynthesis
...
The
chlorotic leaves due to irregular synthesis of chlorophyll or breakdown of chlorophyll
pigment exhibit inefficient photosynthesis
...
Leaf
The leaf characters such as leaf size, chlorophyll content, number of stomata
...
The
maximum photosynthetic activity is usually seen in the physiologically functional and full
size leaves (usually third/fourth leaf from the tip of the shoot system)
...
Carbohydrates
If the accumulated carbohydrates are not translocated, the photosynthetic rate is
reduced and respiration is increased
...
This reduces the effective surface in the chloroplast and the rate of
photosynthesis is decreased
...
Phytohormones
Treharne (1970) reported first that photosynthesis may be regulated by plant hormone
system
...
Meidner (1967) also reported that kinetin @ 3µm causes 12 per cent
increase in photosynthesis within one hour of the treatment
...
Decker (1955) discovered the process and it is also called as C2
cycle as the 2 carbon compound glycolic acid acts as the substrate in photorespiration
...
However in some
plants, the respiration is more in light than in dark
...
Photorespiration is carried out only in the presence of light
...

In photorespiration, temperature and oxygen concentration play an important role
...
The rate of
photorespiration increases with the increase in the concentration of oxygen
...
This kind of respiration is seen in plants like cotton, pulses, capsicum, peas,
tomato, petunia soybean, wheat, oats, paddy, chlorella etc and it is absent in grasses
...
In the presence of excess oxygen and low CO2 , ribulose 1,5 diphosphate produced in the
chloroplast during photosynthesis is split into 2 phospho glycolic acid and 3 phospho
glyceric acid by the enzyme, ribulose 1,5 diphosphate oxygenase
2
...

3
...

4
...
Here, it combines
with oxygen to form glyoxylic acid and hydrogen peroxide
...
Hydrogen peroxide is toxic and it is broken down into water
and oxygen by the enzyme, Catalase
...
In this
process, glycolic acid is converted into carbohydrate and CO2 is released as the by product
...

5
...


6
...
In the
mitochondria, two molecules of glycine condense to form serine and liberate carbon dioxide
and ammonia
...
Amino group is removed from serine to form hydroxyl pyruvic acid in the presence of the
enzyme, transaminase
...
Hydroxy pyruvic acid undergoes reduction with the help of NADH to form glyceric acid in
the presence of enzyme alpha hydroxyl acid reductase
...
Finally, regeneration of 3 phosphoglyceric acid occurs by the phosphorylation of glyceric
acid with ATP
...

10
...
If it enters the
chloroplast, it is converted into carbohydrate by photosynthesis and it is suppressed
nowadays with the increased CO2 content in the atmosphere
...
Photorespiration helps in classifying the plants
Generally, photorespiration is found in C3 plants and absent in C4 plants
...
Carbon dioxide is evolved during the process and it prevents the total depletion of CO2 in
the vicinity of chloroplasts
...
The process causes oxidation of glycolic acid which arises as an unwanted byproduct of
photosynthesis
...

3
...

4
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